Monitoring of the heart's electrical signals—electrocardiogram signals—is utilized to monitor the cardiac cycle of patients in order to detect various conditions and disorders. A typical cardiac cycle as represented by an electrocardiogram signal (EKG) consists of a P wave, a QRS complex, and a T wave—which appear in that order. An EKG specialist is trained to analyze aspects of the EKG signal to identify potential disorders or dangers. The P-wave represents the electrical signal created during atrial depolarization; the QRS complex reflects the rapid depolarization of the right and left ventricles following the atrial depolarization (P-wave); and the T-wave represents the repolarization or recovery of the ventricles. The ventricles are larger muscles than the atria, and therefore the depolarization associated with the QRS complex is typically greater in magnitude than the P-wave depolarization or T-wave repolarization. Analysis of the EKG signal includes measurements related to the timing and magnitude of the components of the EKG signal. For example, heart rate is detected based on the interval between successive peaks of the R portion of the QRS complex (e.g., the R-R interval). The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex and reflects the time the electrical impulse takes to travel from the sinus node through the AV node, and provides an indication of AV node function. Likewise, the QT interval is measured form the beginning of the QRS complex to the end of the T wave. With respect to the measured QT interval, a prolonged QT interval has been identified as a risk factor for a number of conditions, such as acute myocardial infarction and ischemia, hypocalcemia, central nervous system events, hypothermia, hypothyroidism, and as a result of medication.
In a hospital setting, a 6-lead configuration or 12-lead configuration is typically utilized to collect EKG signals and measure the QT interval. However, this type of cardiac monitoring typically only takes place in a hospital for a limited period of time, and typically while the patient is non-ambulatory. It would be desirable to provide long-term monitoring (e.g., multiple days) of cardiac activity—and QT interval detection in particular—in order to detect extreme QT interval events that occur infrequently and/or enable the detection of gradual changes in QT interval. In addition, it would be desirable to monitor a patient's QT interval while the patient is active or engaged in activities such as standing up or walking around. Finally, it would be beneficial to be able to monitor and detect a patient's QT interval without the full complement of six or twelve leads typically used for EKG monitoring in a hospital.